1. Field of the Invention
The present invention relates to an apparatus for protecting components of a press machine from an overload occurring through pressing It is applicable to a two-point type press machine having two connecting rods respectively connecting a slide to a crank shaft.
2. Description of the Related Art
When a press machine receives more than a predetermined number of sheet blanks or a sheet blank having an undesirable thickness and presses them in succession, some components of the press machine receive an overload and thereby might be damaged. To overcome this problem the present applicant develoed a prior-art apparatus disclosed in examined Japanese patent publication No. 42-16388 and in FIG. 3 of the present invention.
As depicted in FIG. 3, a cylinder 2 provided within a slide 1 defines a hydraulic pressure chamber 3. The hydraulic pressure chamber 3 contains a pressure-receipt element 4 reciprocally movable in the cylinder 2. The top surface of the pressure-receipt element 4 is in contact with a spherical bottom end of a connecting rod 5 connecting a crank shaft (not shown) and the slide 1. A downward shoulder surface 6 in the hydraulic pressure chamber 3 and an upward shoulder surface 7 of the pressure-receipt element 4 constitute a shut-off valve or openable seal 8. A chamber 9 disposed above the seal 8 in the cylinder 2 communicates through an oil return line 10 with an oil reservoir 11. The hydraulic pressure chamber 3 receives pressurized oil boosted by a pressure booster 15 from the oil reservoir 11 through a pressurized oil supply line 14 having check valves 12 and 13 therein.
The pressure booster 15 delivers a predetermined pressurized oil into the hydraulic pressure chamber 3 so that a combination of a pressurized air from an air source 19 which is controlled by a changeover valve 18 and a return spring 20 alternately reciprocates a piston 17 of a booster pump 16.
The above-described hydraulic pressure chamber 3, pressure-receipt element 4, seal 8 and oil return line 10 constitute a hydraulic overload-protection means 21.
Rotation of the crank shaft vertically moves the slide 1 by means of the connecting rod 5 to press so that the connecting rod 5 applies a press load to the slide 1 through the pressure-receipt element 4 and pressurized oil contained within the hydraulic pressure chamber 3. The valve body 4, when receiving a downward overload in pressing, descends by this downward overload transmit&ed from the connecting rod 5 against a hydraulic pressure contained in the hydraulic chamber 3. Thereby, the seal 8 is opened and oil escaping through the seal 8 out of the hydraulic pressure chamber 3 returns to the oil reservoir 11 through the oil return line 10. Thus, the pressure-receipt element 4 descends relative to the slide 1 to open the seal 8 and thereby protect the components of the press machine from the overload.
The press machine comprises a two-point type press machine, i.e, press machine with two connecting rods 5 and two hydraulic overload-protection means 21.
FIG. 4 illustrates this prior-art two-point type press machine. Two left-hand and right-hand components, lines etc. of FIG. 4 have respective similar labels of FIG. 3 with letters A or B attached.
In accordance with the prior-art two-point type press machine, when one of left-hand and right-hand hydraulic overload-protection means 21A and 21B operates, a slide 1 tilts if the other of the left-hand and right-hand hydraulic overload-protection means 21A and 21B does not operate concurrently, which may damage a slide gib guiding a vertical movement of the slide 1. Thus, in accordance with the prior-art two-point type press machine, a communication line comprising branched lines 22A and 22B of a pressurized oil supply line 14 connects left-hand and right-hand hydraulic pressure chambers 3A and 3B, so that oil escaping through the opened seal 8 out of one of the hydraulic pressure chambers 3A and 3B returns to the oil reservoir 11 through a corresponding one of oil return lines 10A and 10B and oil contained in the other of hydraulic pressure chambers 3A and 3B also concurrently returns to the oil reservoir 11 through the same route as oil contained in the one of the hydraulic pressure chambers 3A and 3B. Thus, left-hand and right-hand pressure-receipt elements 4A and 4B are essentially concurrently descended relative to the slide 11.
Generally, parts of the underside of the slide 1 cannot receive uniform loads in pressing so that the underside of the slide 1 receives a localized load in response to the shape, dimensions etc. of a work to be pressed. Sensing the localized load concurrently with pressing is required for proper pressing However, the prior-art apparatus of FIG. 4 could not sense the localized load since if it would sense the localized load on the basis of the hydraulic pressures contained in the left-hand and right-hand hydraulic pressure chambers 3A and 3B and changing in pressing, the hydraulic pressure chambers 3A and 3B continuously communicated with each other, as described above, so that hydraulic pressures contained in the hydraulic pressure chambers 3A and 3B continuously equaled each other. In addition, the prior-art apparatus of FIG. 4 has equalized the hydraulic pressures delivered to the hydraulic pressure chambers 3A and 3B. In other words, the prior-art apparatus of FIG. 4 cannot have made different these hydraulic pressures in response to localized loads.
Another prior-art apparatus has been provided in which left-hand and right-hand hydraulic pressure chambers are hydraulically independent of each other and when a hydraulic overload-protection means of one of the hydraulic pressure chambers operates to change the hydraulic pressure contained in the one hydraulic pressure chamber, an electric means senses this change in the hydraulic pressure contained in the one hydraulic pressure chamber to operate a changeover valve connected to the other hydraulic pressure chamber so that the changeover valve allows pressurized oil to be released out of the other hydraulic pressure chamber to reduce the hydraulic pressure contained in the other hydraulic pressure. This prior-art apparatus has entailed a problem in that it produced a time lag between an operation of the one hydraulic overload-protection means and operation of the other hydraulic overload-protection means so that a slide became horizontally oblique to damage a slide gib.